In recent years, to achieve low cost as well as high efficiency of photoelectric conversion devices, thin-film solar batteries that need only a small amount of raw material have attracted attention and have been developed intensively. At present, crystalline thin-film solar batteries have also been developed in addition to the conventional amorphous thin-film solar batteries, and a stacked-type thin-film solar battery referred to as a hybrid solar battery, in which the crystalline and amorphous thin-film solar units are stacked, has also been put into practical use.
The thin-film solar battery generally includes a transparent conductive film, at least one semiconductor thin-film photoelectric conversion unit, and a back electrode stacked in this order on a light-transmissive substrate (also referred to as a transparent substrate). A single photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.
To effectively confine light incident on the transparent substrate side into the photoelectric conversion unit, numerous fine surface concavities and convexities are usually formed on the transparent conductive film. The level difference in the surface unevenness generally falls in a range of approximately 0.05 μm-0.3 μm. To determine the uneven surface shape of the transparent electrode layer, optimal for the thin-film solar battery, there is needed an index that can quantitatively represent the uneven shape. For example, a haze ratio has conventionally been used as the index that can represent the uneven surface shape.
The haze ratio is an index for optically evaluating the surface unevenness of a transparent substrate and is expressed as (diffuse transmittance/total transmittance)×100 [%] (JIS K7136). For measurement of the haze ratio, a haze meter that can automatically measure the haze ratio is commercially available and enables easy measurement. As a light source for the measurement, there is generally used a C light source.
In general, the larger the level difference of the surface unevenness is made, the higher the haze ratio becomes, and thus light having entered the photoelectric conversion unit can more effectively be confined. However, when the surface unevenness has an excessively large level difference, a photoelectric conversion unit cannot be uniformly formed on the uneven surface and then partial deficiencies may be caused in a p-type layer, an i-type layer, or an n-type layer. In such a case, minute current leakage occurs via the deficient parts, causing deterioration in photoelectric conversion properties. Therefore, it is also desired that the surface unevenness of the transparent conductive film is gentle to a certain extent.
As an index that can represent such a surface shape, a surface area ratio (Sdr) is effective. The surface area ratio is also referred to as a Developed Surface Area Ratio, and abbreviated as Sdr. Sdr is defined by Expressions 1 and 2 as follows (K. J. Stout, P. J. Sullivan, W. P. Dong, E. Manisah, N. Luo, T. Mathia: “The development of methods for characterization of roughness on three dimensions”, Publication no. EUR 15178 EN of the Commission of the European Communities, Luxembourg, pp. 230-231, 1994).
                    [                  Mathematical          ⁢                                          ⁢          Expression          ⁢                                          ⁢          1                ]                                                                      S          dr                =                                                            (                                                      ∑                    j                                          M                      -                      1                                                        ⁢                                                            ∑                      k                                              N                        -                        1                                                              ⁢                                          A                      jk                                                                      )                            -                                                (                                      M                    -                    1                                    )                                ⁢                                  (                                      N                    -                    1                                    )                                ⁢                Δ                ⁢                                                                  ⁢                X                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                Y                                                                    (                                  M                  -                  1                                )                            ⁢                              (                                  N                  -                  1                                )                            ⁢              Δ              ⁢                                                          ⁢              X              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              Y                                ×          100          ⁢          %                                    (                  Expression          ⁢                                          ⁢          1                )                                                                                    A                jk                            =                                                1                  2                                [                                                                                                    Δ                        ⁢                                                                                                  ⁢                                                  Y                          2                                                                    +                                                                        {                                                                                    Z                              ⁡                                                              (                                                                                                      x                                    j                                                                    ,                                                                      y                                    k                                                                                                  )                                                                                      -                                                          Z                              ⁡                                                              (                                                                                                      x                                    j                                                                    ,                                                                      y                                                                          k                                      +                                      1                                                                                                                                      )                                                                                                              }                                                2                                                                              +                                                                                                                                                                                    Δ                      ⁢                                                                                          ⁢                                              Y                        2                                                              +                                                                  {                                                                              Z                            ⁡                                                          (                                                                                                x                                                                      j                                    +                                    1                                                                                                  ,                                                                  y                                  k                                                                                            )                                                                                -                                                      Z                            ⁡                                                          (                                                                                                x                                                                      j                                    +                                    1                                                                                                  ,                                                                  y                                                                      k                                    +                                    1                                                                                                                              )                                                                                                      }                                            2                                                                      ]                            ×                                                                                          1                2                            [                                                                                          Δ                      ⁢                                                                                          ⁢                                              X                        2                                                              +                                                                  {                                                                              Z                            ⁡                                                          (                                                                                                x                                  j                                                                ,                                                                  y                                  k                                                                                            )                                                                                -                                                      Z                            ⁡                                                          (                                                                                                x                                                                      j                                    +                                    1                                                                                                  ,                                                                  y                                  k                                                                                            )                                                                                                      }                                            2                                                                      +                                                                                                                                              Δ                    ⁢                                                                                  ⁢                                          X                      2                                                        +                                                            {                                                                        Z                          ⁡                                                      (                                                                                          x                                j                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      )                                                                          -                                                  Z                          ⁡                                                      (                                                                                          x                                                                  j                                  +                                  1                                                                                            ,                                                              y                                                                  k                                  +                                  1                                                                                                                      )                                                                                              }                                        2                                                              ]                                                          (                  Expression          ⁢                                          ⁢          2                )            
Here, M and N represent the numbers obtained by dividing an entire measurement region by a minute length ΔX in the x direction and a minute length ΔY in the y direction, respectively. Z(x, y) represents a height at a prescribed position (x, y) in a minute measurement region ΔXΔY. Sdr represents an increase ratio of an actual surface area to a flat area in the entire measurement region. In other words, the larger and sharper the unevenness is, the larger the value of Sdr becomes. Sdr can be measured with a scanning microscope such as an AFM (atomic force microscope) or an STM (scanning transmission microscope).
The i-type layer, which is substantially an intrinsic semiconductor layer, occupies a large part of thickness of the photoelectric conversion unit, and photoelectric conversion mainly occurs in the i-type layer. Therefore, the i-type layer is usually referred to as an i-type photoelectric conversion layer or simply referred to as a photoelectric conversion layer. The photoelectric conversion layer is not limited to the intrinsic semiconductor layer, and it may be a slightly doped p-type or n-type layer as long as loss of light absorbed by impurities (dopant) is not problematic. While the photoelectric conversion layer preferably has a larger thickness for its better light absorption, an excessive increase of the thickness inevitably increases cost and time for depositing the layer.
On the other hand, the conductivity-type semiconductor layers of a p-type and an n-type play a role of generating an internal electric field in the photoelectric conversion unit, and the intensity of the internal electric field influences the value of open-circuit voltage (Voc) that is one of the important properties of the thin-film solar battery. However, these conductivity-type semiconductor layers are inactive layers that do not directly contribute to photoelectric conversion, and light absorbed by impurities with which the conductivity-type semiconductor layers are doped becomes loss that cannot contribute to power generation. Accordingly, the conductivity-type semiconductor layers of the p- and n-types preferably have their thicknesses as small as possible on condition that they can generate a sufficient internal electric field. Generally, each of the conductivity-type semiconductor layers is approximately at most 20 nm in thickness.
Here, when a photoelectric conversion unit or a thin-film solar battery includes an amorphous photoelectric conversion layer occupying the substantial part thereof, it is referred to as an amorphous photoelectric conversion unit or an amorphous thin-film solar battery, and when it includes a crystalline photoelectric conversion layer, it is referred to as a crystalline photoelectric conversion unit or a crystalline thin-film solar battery, regardless of whether the conductivity-type semiconductor layers of the p- and n-types included therein are amorphous or crystalline.
As a method of improving conversion efficiency of the thin-film solar battery, there is a method of stacking at least two photoelectric conversion units. In this case, a front unit including a photoelectric conversion layer of a large band gap is disposed on the light incident side of the thin-film solar battery, and rear units each including a photoelectric conversion layer of a smaller band gap are disposed in descending order of the band gap on the rear side of the front unit, so that photoelectric conversion becomes possible in a wide wavelength range of incident light and thus conversion efficiency of the entire solar battery can be improved. Among such stacked-type solar batteries, particular one referred to as a silicon hybrid solar battery includes an amorphous silicon-based photoelectric conversion unit and a crystalline silicon-based photoelectric conversion unit stacked and electrically connected in series.
For example, the wavelength of light that can be photoelectrically converted by amorphous i-type silicon is up to approximately 800 nm on the longer-wavelength side, while the wavelength of light that can be photoelectrically converted by crystalline i-type silicon is longer and up to approximately 1150 nm.
Among output properties of the silicon hybrid solar battery, short-circuit current density (Jsc) is influenced by the relation between a spectral sensitivity integral current of the amorphous silicon-based photoelectric conversion unit disposed on the front side (hereinafter referred to as a “top cell”) and a spectral sensitivity integral current of the crystalline silicon-based photoelectric conversion unit disposed on the rear side (hereinafter referred to as a “bottom cell”). Here, the spectral sensitivity integral current means output current density calculated by integrating the product of measured spectral sensitivity of the photoelectric conversion unit and spectral intensity of sunlight typically represented as air mass 1.5, with respect to each wavelength. More specifically, if the spectral sensitivity integral current of the bottom cell is larger than that of the top cell, Jsc of the entire solar battery is limited to the spectral sensitivity integral current of the top cell. In contrast, if the spectral sensitivity integral current of the bottom cell is smaller, Jsc of the entire solar battery is limited to the spectral sensitivity integral current of the bottom cell.
An amorphous silicon-based photoelectric conversion layer generally having a large light absorption coefficient can obtain sufficient Jsc even if it has a thickness of approximately at most 0.3 μm for light absorption. However, in order that a crystalline silicon-based photoelectric conversion layer generally having a small light absorption coefficient can sufficiently absorb light of a longer wavelength as well, it desirably has a thickness of approximately 1.5-3 μm. In other words, it is usually desired that the crystalline silicon-based photoelectric conversion layer should have a thickness approximately 5-10 times as large as a thickness of the amorphous silicon-based photoelectric conversion layer. Similarly, in the silicon hybrid solar battery as well, it is also desired to maintain the thickness ratio of the crystalline photoelectric conversion layer to the amorphous photoelectric conversion layer to be approximately 5-10 (see Patent Document 1, i.e. Japanese Patent Laying-Open No. 2001-177134 and Patent Document 2, i.e. Japanese Patent Laying-Open No. 2001-308362). If the thickness of the crystalline photoelectric conversion layer is made smaller with respect to this relation, Jsc of the entire solar battery will be a value limited to the spectral sensitivity integral current of the bottom cell.
Patent Document 1: Japanese Patent Laying-Open No. 2001-177134
Patent Document 2: Japanese Patent Laying-Open No. 2001-308362